One of the most effective and efficient methods of deploying high-speed digital services to business and residential customers is to use one of the many forms of DSL (Digital Subscriber Loop) technologies over the copper wires originally used for delivering telephone service. This approach has become very popular in the last 20 years due to the fact that copper wires are already deployed almost everywhere, and are quite easy to access at the Central Office (CO), at the cabinet or Remote Terminal (RT), and at the customer premises (CP).
In typical DSL deployments, a communication link is established between two transceivers connected to each other by a copper wire pair, i.e., two copper wires twisted together. One transceiver is located either at the CO or at the RT, and will be referred to here as the Network Node Equipment (NNE) transceiver, and the other transceiver is located at the customer location, and will be referred to here as the Customer Premise Equipment (CPE) transceiver. Typically, a number of copper pairs may be enclosed in a single cable.
One of the main limitations of DSL technology is the significant spectral interference between DSL services deployed on different twisted copper pairs in the same cable. Spectral interference between different high-bitrate services in a copper cable is caused by the fact that each copper pair acts as an antenna. The signal transmitted on each copper pair, which is intended for the receiver located at the other end of that copper pair, is also inadvertently picked up by receivers connected to neighboring copper pairs, because those pairs are not individually shielded from each other. These inadvertently received signals appear as additional noise on the receivers of neighboring pairs, thereby reducing the Signal-to-Noise Ratio (SNR) and corresponding data capacity of those receivers. This creates the well-known phenomenon of “crosstalk”, aptly named for the effect it caused in the early days of the telephone, when the telephone discussion taking place on one line could sometimes be overheard by the people conversing on a different line.
Due to the physical characteristics of copper pairs, and in particular due to the average length of the twist between the two copper wires making up each pair, the crosstalk coupling between different pairs increases exponentially with the frequency of the transmitted signal. But this crosstalk coupling between copper pairs is only one of the three factors that determine the strength of crosstalk; the other two are the strength of the disturbing transmitter (typically referred to as a “disturber”) and the sensitivity of the disturbed receiver (typically referred to as a “victim”) at any given frequency. For example, if the frequency band of the signals transmitted by the disturber is different than the frequency band of the signals received by the victim, then there will typically be almost no crosstalk, and therefore no impact on the data capacity of the victim.
Crosstalk typically consists of Near-End Crosstalk (NEXT), caused by disturbers located at the “near-end”, i.e., on the same side (network side or customer side) of the copper loop as the victim's receiver, and Far-End Crosstalk (FEXT), caused by disturbers located at the “far-end”, i.e., on the opposite side of the copper loop from the victim's receiver.
The severe deterioration in the data capacity of DSL services due to crosstalk has sparked significant innovation in crosstalk-reduction methods. These methods typically aim to reduce self-crosstalk and/or alien crosstalk, which for the purposes of this document are defined as follows: “Self-crosstalk”, consisting of Self-NEXT and Self-FEXT, is generated by transmitters connected to other lines that are physically connected to the same DSL equipment as the victim's line on at least one side of the loop (i.e., on the NNE side or the CPE side). “Alien crosstalk”, consisting of Alien NEXT and Alien FEXT, is generated by transmitters connected to lines that are physically connected to different DSL equipment than the victim's line on both sides of the loop (i.e., on both the NNE side and the CPE side).
In the case of self-crosstalk, the fact that the disturbing lines and disturbed lines are connected to the same DSL equipment implies that the signals transmitted on the disturbing lines are known. This means that the disturbed lines can take advantage of that knowledge to essentially cancel self-crosstalk. This basic principle has been implemented in various forms of self-crosstalk cancellation, for example in Self-NEXT cancellation for Gigabit Ethernet and SHDSL (Single-pair High-Speed DSL). The technology of interest in this case, however, is VDSL (Very-high-speed DSL), which uses different frequency bands for upstream and downstream transmission. Therefore, lines that carry VDSL services do not generate Self-NEXT with respect to each other.
The data capacity of copper wires decreases significantly as the length of the copper loop increases. Therefore, in order to deliver very high data rates, VDSL services are typically deployed on very short loops. The length of the copper loops is typically decreased by deploying the NNE in a cabinet or RT that is connected to the CO via a fiber link. The customers located close to the RT are connected to the NNE over their shortened copper loops.
Since FEXT emanates from disturbers located on the opposite side of the copper loop from the victim's receiver, its effect also decreases significantly as the length of the copper loop increases. Therefore, on the short loops used by VDSL services, Self-FEXT is not attenuated and becomes a significant concern.
In recent years, the concept of “vectoring” has been developed for cancelling Self-FEXT between VDSL links that are connected to the same VDSL access multiplexer (VDSLAM) on the NNE side but different VDSL modems on the CPE side. This means that vectoring can be used to cancel Self-FEXT between DSL lines serving different customer locations, as long as they originate from the same VDSLAM on the NNE side of the loop. Vectoring utilizes the fact that all the disturbing signals are available in the NNE location, either on the transmitter side for downstream signals, or on the receiver side for upstream signals. Therefore, downstream Self-FEXT can be cancelled by precoding the transmitted signals with crosstalk-cancelling additional signals, and upstream Self-FEXT can be cancelled by decoding the received signals and subtracting the crosstalk effects of each of those received signals from the other received signals. Vectoring has already been standardized by the International Telecommunications Union (ITU), and offers great promise for significant improvement of data rates on VDSL link.
Unfortunately, carriers across the world appear hesitant to deploy vectoring in their networks. One principal concern is due to the inability of vectoring to deal with the effects of Alien FEXT. In the case of alien crosstalk, the modems on the victim lines do not have access to the signals transmitted on the disturber lines. Therefore, alien crosstalk cannot be cancelled, as with Self-FEXT. However, the effects of alien crosstalk can be mitigated by correlating the received crosstalk noise across multiple receivers, and removing the correlated part of the noise from at least some of these receivers, thereby increasing the data capacity of the corresponding lines. This type of correlation-based scheme can result in noticeable performance benefits as long as the number of crosstalk sources (i.e., the number of disturbing alien transmitters) is lower than the number of disturbed receivers whose noises are correlated.
The problem with known correlation-based alien crosstalk mitigation methods is that they are not effective in typical VDSL deployments. Consider the simple example illustrated in FIG. 1, in which a single cabinet 1100 contains two VDSLAMs with vectoring, 1110 and 1120, deployed by two different carriers. Each of customers 1400A, 1400B, 1400C, and 1400D, may be connected to a VDSLAM by a twisted copper pair, e.g., 1212, 1214, 1222, and 1224), which run through cables 1210 and 1220. A switch, 1130 is connected to VDSLAMs 1110 and 1120.
Since each customer is free to select either of the two carriers to deliver VDSL service to their home, each of the two VDSLAMs 1110 and 1120 could be connected to any customer 1400A-D in the vicinity of the cabinet 1100. Therefore, any VDSL link operated by one carrier out of its own VDSLAM is likely to be affected by alien crosstalk generated by VDSL links operated by the other carrier out of the other VDSLAM. For example, customer 1400A, which is served by VDSLAM 1110, delivered on twisted copper pair 1212 located in cable 1210 is affected by downstream FEXT 1343 by and upstream FEXT 1346 from twisted copper pair 1214. However, since the VDSL link on copper pair 1214 is served by VDSLAM 1120, it is alien FEXT. The alien FEXT 1343 and FEXT 1346 cannot be cancelled by VDSLAM 1110 because the signals are not known to VDSLAM 1110. Similarly, for example, customer 1400D, which is served by VDSLAM 1120, delivered on twisted copper pair 1224 located in cable 1220 is affected by downstream FEXT 1353 by and upstream FEXT 1356 from twisted copper pair 1222. However, since the VDSL link on copper pair 1222 is served by VDSLAM 1110, it is alien FEXT. The alien FEXT 1353 and FEXT 1356 cannot be cancelled by VDSLAM 1120 because the signals are not known to VDSLAM 1120.
In the downstream direction, i.e., from the NNE to the CPE, the victim receivers are located at the CPE, and the vast majority of VDSL links use only one such CPE receiver. Therefore, there are no multiple receivers across which the alien crosstalk signals could be correlated and their effect mitigated. Even in the rare cases where there are two, three, or even four receivers located at the CPE (for example, in the case of bonding multiple copper pairs together to deliver even higher data rates), the number of disturbing alien crosstalk transmitters is in most cases equal to or higher than the number of collocated CPE receivers, thereby negating the benefits of alien crosstalk mitigation.
In the example of FIG. 1, the presence of alien FEXT between VDSLAMs 1110 and 1120 implies that vectoring will on average be able to cancel only about half of the disturbers for each victim (since the other half are likely to be connected to the other VDSLAM), resulting in an approximate SNR gain of 3 dB. This SNR gain is much lower than typical SNR gains of 15-30 dB that can be achieved by vectoring in the absence of alien FEXT.
The above example illustrates the dramatic reduction in the benefits of vectoring that can result from alien FEXT. What this means to carriers is that they might go through the significant effort and expense of upgrading their VDSL equipment to support vectoring, only to see the resulting benefits disappear when a different carrier deploys another VDSLAM out of the same cabinet.
The same problem may arise even when both VDSLAMs 1110 and 1120 belong to the same carrier, because technology for vectoring across multiple VDSLAMs has not yet been developed. Moreover, even if it is eventually developed, it may prove to be too expensive and too cumbersome to deploy.
One proposed remedy for the alien crosstalk where both VDSLAMs are operated by the same carrier, is so-called “binder management.” To better understand this concept, let us examine the nature of crosstalk in telephone wires. As previously mentioned, one of the three factors that determine the strength of crosstalk between a disturber and a victim is the crosstalk coupling between the two corresponding copper pairs, namely the copper pair connected to the disturber and the copper pair connected to the victim. Copper pairs are arranged in bundles referred to as “binders”, with each binder typically containing 10-50 copper pairs. A cable may contain just a single binder, or it may contain multiple binders. The total number of copper pairs in a cable depends greatly on the location from which the cable is deployed. Cables originating from the CO may contain hundreds or even thousands of copper pairs, while cables originating from the cabinet typically contain no more than one hundred copper pairs.
The key difference between cables and binders from the point of view of crosstalk has to do with the shielding method: cables are almost always wrapped in metal shielding that essentially eliminates any crosstalk coupling between copper pairs in different cables. Binders, on the other hand, are not individually shielded, and therefore the crosstalk coupling between copper pairs in different binders that belong to the same cable may be as strong as or even stronger than the crosstalk coupling between copper pairs in the same binder.
Thus, in the example of FIG. 1, copper pairs 1212 and 1214 belong to the same cable 1210, and therefore they generate crosstalk 1343 and 1346 to each other. Similarly, copper pairs 1222 and 1224 belong to the same cable 1220, and therefore they generate crosstalk 1353 and 1356 to each other. However, there is essentially no crosstalk generated from copper pairs 1212 and 1214 to copper pairs 1222 and 1224 or vice versa, since those copper pairs belong to different cables.
“Binder management” refers to the process of connecting copper pairs with strong crosstalk couplings to the same VDSLAM. The rationale behind this process is that since FEXT cannot be cancelled across different VDSLAMs, it is preferable to connect all the copper pairs that generate crosstalk to each other to the same VDSLAM.
FIG. 2 illustrates the process of binder management, where all the pairs from each cable are connected to respectively separate VDSLAMs. In this example, each of customers 2400A, 2400B, 2400C, and 2400D, may be connected to a VDSLAM by a twisted copper pair, e.g., 2212, 2214, 2222, and 2224), which run through cables 2210 and 2220. According to the binder management arrangement, all pairs from cable 2210, e.g., cables 2212 and 2214, are connected to VDSLAM 2110, and all the pairs from cable 2220, e.g., cables 2222 and 2224, are connected to VDSLAM 2120. A switch, 2130 is connected to VDSLAMs 2110 and 2120.
It will be observed that in the system of FIG. 2, by virtue of the binder management arrangement, alien crosstalk is eliminated. For example, customer 2400A, which is served by VDSLAM 2110, delivered on twisted copper pair 1212 located in cable 1210 is affected by downstream FEXT 2343 by and upstream FEXT 2346 from twisted copper pair 2214. Since the VDSL link on copper pair 1214 is served by the same VDSLAM, i.e., VDSLAM 2110, it is self-FEXT, and therefore, may be cancelled by VDSLAM 2110 using vectoring, because the signals are known to VDSLAM 2110. Similarly, for example, customer 2400D, which is served by VDSLAM 2120, delivered on twisted copper pair 2224 located in cable 2220 is affected by downstream FEXT 2353 by and upstream FEXT 2356 from twisted copper pair 2222. Again, since the VDSL link on copper pair 2222 is served by the same VDSLAM, i.e., VDSLAM 2120, it is self-FEXT, and may be cancelled by VDSLAM 2120 using vectoring, because the signals are known to VDSLAM 2120.
Unfortunately, this process is currently not feasible for carriers for at least two reasons. First, the technician connecting a new VDSL customer would have to figure out which of the existing VDSL customers would be disturbed by the new VDSL link, and then decide which of the two or more VDSLAMs should be used to connect the new customer. However, this information is not typically available to a technician in the field. Second, even if this were possible, DSL technicians commonly move customers to different pairs in order to correct problems with the service. Therefore, each time a technician wanted to move a customer to a different pair, he would have to figure out which VDSLAM the new pair should be connected to, and then re-provision the service of the customer out of the new VDSLAM, resulting in significantly higher operating expenses for the carriers, and much longer service disruptions for the customers.
Therefore, it is desirable to find a solution to the problem of alien FEXT that does not involve binder management or the use of vectoring across different VDSLAMs.